Chapter 8 Fall 2011 (1) - Process and Measurement System...

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1 Process and Measurement System Capability Analysis
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2 Process Capability Analysis Histograms Probability Plots PCR CC for Process Capability Gage Capability Studies Spec Limits for Discrete Components Natural Tolerance Limits
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8-1. Introduction Process capability analysis is an engineering study to estimate process capability. In a product characterization study, the distribution of the quality characteristic is estimated.
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4 Process Capability Uniformity of the process, specifically it’s variability natural variability - instantaneous variability variability over time. Natural tolerance limits 0.27% out, but large 2700/1,000,000 think of plane crashes! If process outcome is non-normal, then this value may be larger σ μ 3 3 - = + = LNTL UNTL
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5 Process Capability Analysis Usually measuring functional parameters on the product not necessarily the process best to have the process not always available - product characterization Data used for: predicting how well the process will hold the tolerances assist product developers in process development determine sampling intervals vendor selection reduce variability Techniques histograms/probability plots CC DOE PCR
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6 Histograms – at least 100 observations Select machine(s) Select process operating conditions define conditions, documentation is important for future reference and repeatability Select operator their variability Monitor data collection Immediate and visual impression of process performance note shape, center and spread Stem-and-Leaf plots are alternatives
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Chapter 8 7 Process may have good potential capability Reasons for Poor Process Capability
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8 Probability Plotting Use of normal probability plots See chapter 3 Mean = 50 th percentile Standard Deviation = 84 th -50 th percentiles Can estimate process yields and fallouts Careful in interpretation. Sometimes misleading Best to use formal goodness of fit tests Shapiro Careful with certain distribution fitting functions. Really does take some knowledge of the data and process Note β 1 and β 2 regions on Figure 8-6, where skewness and kurtosis are measured for various distributions
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Probability Plots We are often interested in whether data come from a population distribution of a particular type One tool to check assumptions - Probability Plot . Expected percentiles of the distribution are plotted versus actual sample data points. If the distribution is correct then the points will be in an approximately straight line, otherwise the pattern of points will depart from linearity.
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This note was uploaded on 02/14/2012 for the course IE 530 taught by Professor Ravindran during the Spring '97 term at Purdue University-West Lafayette.

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Chapter 8 Fall 2011 (1) - Process and Measurement System...

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